[PATCH 0/4] Use pointer arithmetic for array references [PR102043]

[Mikael Morin <mikael@gcc.gnu.org>]

Hello,

this is a fix for PR102043, which is a wrong code bug caused by the
middle-end concluding from array indexing that the array index is
non-negative.  This is a wrong assumption for "reversed arrays",
that is arrays whose descriptor makes accesses to the array from
last element to first element.  More generally the wrong cases are
arrays with a descriptor having a negative stride for at least one 
dimension.

I have been trying to fix this by stopping the front-end from generating
bogus code, by either stripping array-ness from descriptor data
pointers, or by changing the initialization of data pointers to point
to the first element in memory order instead of the first element in
access order (which is the last in memory order for reversed arrays).
Both ways are very impacting changes to the frontend and I haven’t
been able to eliminate all the regressions in time using either way.

However, Richi showed with a patch attached to the PR that array
references are crucial for the problem to appear, and everything works
if array indexing is replaced with pointer arithmetic.  This is
much simpler and doesn’t imply invasive changes to the frontend.

I have built on top of his patch to keep the array indexing in cases
where the change to pointer arithmetic is not necessary, either because
the array is not a fortran array with a descriptor, or because it’s
known to be contiguous.  This has the benefit of reducing the churn
in the dumped code patterns used in the testsuite.  It also avoids
ICE regression such as interface_12.f90 or result_in_spec.f90, but
I can’t exclude that those could be a real problem made latent.

Patches 1 to 3 are preliminary changes to avoid regressions.  The main
change is number 4, the last in the series.

Regression tested on x86_64-pc-linux-gnu.  OK for master?

Mikael Morin (4):
  fortran: Pre-evaluate string pointers. [PR102043]
  fortran: Update index extraction code. [PR102043]
  fortran: Generate an array temporary reference [PR102043]
  fortran: Use pointer arithmetic to index arrays [PR102043]

 gcc/fortran/trans-array.cc                    |  60 +++++-
 gcc/fortran/trans-expr.cc                     |   9 +-
 gcc/fortran/trans-io.cc                       |  48 ++++-
 gcc/fortran/trans.cc                          |  42 +++-
 gcc/fortran/trans.h                           |   4 +-
 .../gfortran.dg/array_reference_3.f90         | 195 ++++++++++++++++++
 gcc/testsuite/gfortran.dg/c_loc_test_22.f90   |   4 +-
 gcc/testsuite/gfortran.dg/dependency_49.f90   |   3 +-
 gcc/testsuite/gfortran.dg/finalize_10.f90     |   2 +-
 .../gfortran.dg/negative_stride_1.f90         |  25 +++
 .../gfortran.dg/vector_subscript_8.f90        |  16 ++
 .../gfortran.dg/vector_subscript_9.f90        |  21 ++
 12 files changed, 401 insertions(+), 28 deletions(-)
 create mode 100644 gcc/testsuite/gfortran.dg/array_reference_3.f90
 create mode 100644 gcc/testsuite/gfortran.dg/negative_stride_1.f90
 create mode 100644 gcc/testsuite/gfortran.dg/vector_subscript_8.f90
 create mode 100644 gcc/testsuite/gfortran.dg/vector_subscript_9.f90

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2.35.1